The invention pertains to light sources and particularly to light sources used in interferometric fiber optic gyroscopes (IFOG's). More particularly, the invention pertains to fiber light sources (FLS).
The conventional architecture of a backward-pumped FLS includes a 1480 nanometer (nm) pump. The 1480 nm pump laser is used to optically pump the doped (e.g., Erbium) fiber. The output spectrum of the FLS with a 1480 nm pump has a significant 1560 nm emission band. The 1480 nm pump may be replaced by a more inexpensive 980 nm pump. However, the resulting FLS does not produce light with an acceptable output spectrum and thus a coherence function to permit an IFOG with the latter FLS to perform as well as an IFOG with a 1480 nm pump. The output spectrum of the FLS with a 980 nm pump has a very prominent 1530 nm emission band that can be strong compared to longer emission wavelengths including the 1556 nm band. The 1532 nm emission is much more sensitive to pump power and pump perturbations than the longer wavelength constituents of the spectrum. The resulting degradation in the coherence of the FLS adversely affects the spectral stability of the FLS and the bias stability of the IFOG. Spectral instabilities of the light adversely affect the scale factor stability in the IFOG, which is unacceptable in a navigation grade IFOG.
The 980 nm pump laser of the FLS needs to be specified to a very tight tolerance to provide satisfactory wavelength stability and wavelength repeatability at the FLS output. The wavelength stability and repeatability have a direct impact on the scale factor of the IFOG. Considering all of the related FLS dependencies of the parameters on the environmental temperature, the pump laser wavelength needs to be specified (at 975-985 nm to within plus and minus 0.5 nm) to match the EDF such that the output wavelength perturbation is minimal relative to pump wavelength perturbation (or change or deviation). This wavelength tolerance is ten times more narrow than for telecommunication applications. Although, in principle, one can set the pump operating wavelength by finding the correct temperature of the thermo electric cooler (TEC) which contains the pump, and operating the pump laser at this temperature. A related requirement for wide temperature operation on the total power dissipation of the pump, limits the TEC operating temperature between 30 and 35 degrees Centigrade (C.). The wavelength tolerance and power dissipation of the pump are two constraints that increase process and assembly complexity and decreases yield in pump manufacturing. The result is a high cost pump.